std::setbuf

 

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FILE fpos_t	stdinstdoutstderr
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Defined in header <cstdio>
void setbuf( std::FILE* stream, char* buffer );

Sets the internal buffer to use for I/O operations performed on the C stream stream.

If buffer is not null, equivalent to std::setvbuf(stream, buffer, _IOFBF, BUFSIZ).

If buffer is null, equivalent to std::setvbuf(stream, nullptr, _IONBF, 0), which turns off buffering.

Parameters

stream	-	the file stream to set the buffer to
buffer	-	pointer to a buffer for the stream to use. If a null pointer is supplied, the buffering is turned off. If not null, must be able to hold at least BUFSIZ characters

Return value

(none)

Notes

If BUFSIZ is not the appropriate buffer size, std::setvbuf can be used to change it.

std::setvbuf should also be used to detect errors, since std::setbuf does not indicate success or failure.

This function may only be used after stream has been associated with an open file, but before any other operation (other than a failed call to std::setbuf/std::setvbuf).

A common error is setting the buffer of stdin or stdout to an array whose lifetime ends before the program terminates:

int main()
{
    char buf[BUFSIZ];
    std::setbuf(stdin, buf);
} // lifetime of buf ends, undefined behavior

Example

std::setbuf may be used to disable buffering on streams that require immediate output.

Run this code

#include <chrono>
#include <cstdio>
#include <thread>
 
int main()
{
    using namespace std::chrono_literals;
 
    std::setbuf(stdout, nullptr); // unbuffered stdout
    std::putchar('a'); // appears immediately on unbuffered stream
    std::this_thread::sleep_for(1s);
    std::putchar('b');
}

Output:

ab

See also

setvbuf	sets the buffer and its size for a file stream (function)
C documentation for setbuf